1. Field of the Invention
This application is directed to a method of testing a closed hydraulic system for example a blowout preventer (BOP) assembly for leaks. Oil and Gas Exploration risk management includes the ability to control subsurface pressure which may be encounted during drilling operation. The primary mechanism utilized by operators to control downhole pressure is the hydrostatic pressure as a result of the drilling fluid contained within the wellbore. The drilling fluid is engineered and formulated to a density that provides a hydrostatic pressure inside of the wellbore that is greater than the formation pressure being drilled. In the majority of drilling operations, the hydrostatic control is adequate. However, from time-to-time the operator may encounter a higher than expected formation pressure where there is not adequate hydrostatic pressure to control the wellbore pressure. During these times the operator relies on a series of mechanical controls to stabilize the wellbore and prevent a “Blow Out.” A blow out is the uncontrolled release of fluid or gas from the wellbore. This event is extremely dangerous and therefore must be avoided if at all possible. The primary mechanical control device utilized by operators to control wellbore pressure is the Blowout Preventer (BOP) assembly. The BOP assembly consists of valves, and multiple sealing and shearing devices that are hydraulically actuated to provide various means of sealing around the drill string or shearing it off entirely, completely sealing the wellbore. It is essential that the BOP assembly operate as designed during these critical operations. Therefore it is a regulatory requirement to test the functionality and the integrity of the BOP assembly before starting drilling operations and at specific events during the drilling operations.
2. Description of Related Arts Invention
The BOP assembly test is a series of pressure tests at a minimum of two pressure levels, low pressure and high pressure. During the pressure test, fluid from a high pressure pump unit is introduced into the closed BOP assembly in a volume sufficient to cause the internal pressure within the closed BOP assembly to rise to the first pressure test level. Once the first pressure test level is established the high pressure pump system is isolated from the closed BOP assembly and the pressure is monitored, utilizing electronic or mechanical chart recorders, for a specified time period. During the monitoring phase the pressure decay is determined and compared to the pressure decay specification. A typical specification for compliance allows for a pressure decay rate of no more than 5 psi/minute or 25 psi total over the entirety of the five minute test. Measuring leak rate utilizing the indirect result of pressure decay, while widely accepted, is problematic and not indicative of a specific leak rate. Such factors as the compressibility, volume of the required intensification fluid, the amount of trapped air within the BOP assembly, and the flexibility of the BOP assembly have an effect on the relationship between the pressure decay rate and the actual leak rate. An example related to trapped air: if a typical land-based BOP assembly having an approximate test volume of 15 gallons and a volumetric loss rate of approximately 25 cc/min @ 250 psi is tested at 250 psi with approximately 7.5 gallons of air trapped within the BOP assembly during the monitoring phase of the hydrostatic test and then subsequently tested with approximately 2.5 gallons of air trapped within the BOP assembly during the monitoring phase of the hydrostatic test, the BOP would pass the first test with approximately a 3.2 psi/min pressure decay rate but, it would fail the second test with a 7.4 psi/min pressure decay rate. Each test would have the same volumetric loss rate of 25 cc/min but the result of the tests would be significantly different. In another example related to compressibility: if a typically BOP assembly, having 5 gallons of trapped air within the BOP assembly, and rig up configuration requiring approximately 50 gallons of a typical test fluid to conduct a high pressure (5000 psi) pressure decay test is first tested with a volumetric loss rate of approximately 3 cc/min, the resultant approximate psi/min decay rate will be 6.0 psi/min and the test would fail. If the same high pressure (5000 psi) pressure decay test is applied to the same typical BOP assembly but the rig up configuration requires approximately 100 gallons of a typical test fluid to conduct a pressure decay test and has the same 3 cc/min volumetric loss rate, then the approximate psi/min decay rate will be 3.0 psi/min and the test would pass. Each test would have the same volumetric loss rate and, related destructive energy, but the result of the tests would be significantly different. It is very important to realize that volumetric leak rate indicators, in conjunction with the pressure at which the leaks are occurring, are reliable indicators of the destructive energy dissipated across the leak paths, while psi/min decay rate indicators are not reliable indicators of the destructive energy dissipated across the leak paths. This is evident in the above example where in each test the pressure of the test and the volumetric loss rate are the same, and therefore so is the destructive energy, but the psi/min decay rate for each test is different and therefore not indicative of the destructive energy related to the leaks.
Thus there remains a need for a hydrostatic test method that provides for a means of directly correlating a psi/min decay rate to a measured leak rate when performing hydrostatic testing of BOP assemblies or other pressure vessels such as pipe lines and tanks.